The invention relates to an anionically polymerized, impact-modified polystyrene with a disperse soft phase which comprises particles having capsule particle morphology, and also to a process for its preparation.
Various continuous or batch processes, in solution or suspension, are known for producing impact-modified polystyrene, as described in Ullmanns Enzyklopxc3xa4die, Vol. A21, VCH Verlagsgesellschaft Weinheim 1992, pp. 615-625. These processes dissolve a rubber, usually polybutadiene, in monomeric styrene, and polymerize the styrene by a free radical mechanism via thermal or peroxidic initiation. Alongside the homopolymerization of styrene, graft polymerization of styrene on polybutadiene also takes place. The simultaneous processes of polystyrene formation and consumption of monomeric styrene cause xe2x80x9cphase inversionxe2x80x9d. The properties of the impact-modified polystyrene are determined by the morphology, size and size distribution of the disperse rubber particles. These depend on various process parameters, such as the viscosity of the rubber solution and the shear forces arising during stirring.
The process parameters known from free-radical preparation of impact-modified polystyrene cannot be directly transferred to the anionic polymerization of styrene in the presence of rubbers, since the reaction mechanisms for free-radical and anionic polymerization of styrene are different. For example, it is not possible to use homopolybutadiene alone, since no graft reactions occur during anionic polymerization of styrene.
DE-A 42 35 978, DE-A-42 35 978, WO 96/18666, WO 96/18682, WO 99/40135 or U.S. Pat. No. 4,153,647, for example, disclose a process for preparing thermoplastic molding compositions by anionically polymerizing styrene in the presence of styrene-butadiene block copolymers. The resultant impact-modified products have lower residual monomer contents and lower oligomer contents than do products obtained by free-radical polymerization. The disperse soft phase generally develops cellular particle morphology.
WO 98/07766 describes the continuous preparation of impact-modified molding compositions using styrene-butadiene rubbers. The rubbers were polymerized anionically using alkyl compounds of alkaline earth metals, of zinc and of aluminum, in styrene as solvent. However, their butadiene blocks always contain small amounts of copolymerized styrene.
WO 99/67308 describes anionically polymerized impact-modified polystyrene with high stiffness and toughness and with cellular particle morphology.
It is an object of the present invention to provide an anionically polymerized, impact-modified polystyrene with high gloss, and also a process for its preparation.
We have found that this object is achieved by means of an anionically polymerized, impact-modified polystyrene which comprises a disperse soft phase with particles having capsule particle morphology.
It is preferable for at least 90 percent by volume, in particular at least 95 percent by volume, of the soft phase to be composed of particles with capsule particle morphology.
The impact-modified polystyrene of the invention may be obtained by anionically polymerizing styrene in the presence of a styrene-butadiene two-block copolymer and of an anionic polymerization initiator, the styrene-butadiene two-block copolymer having a styrene block content of from 40 to 60% by weight, preferably from 45 to 55% by weight.
For this purpose it is particularly preferable to use styrene-butadiene two-block copolymers whose styrene block S has a weight-average molar mass Mw of from 20,000 to 200,000 g/mol and whose butadiene block B has a weight-average molar mass Mw of from 30,000 to 300,000 g/mol. The transitions between the blocks S and B may be either sharp or blurred.
The anionic polymerization permits impact-modified polystyrene to be obtained with less than 50 ppm of monomeric styrene, in particular less than 10 ppm. Anionically polymerized impact-modified polystyrene generally comprises no cyclic oligomers.
Impact-modified polystyrenes with a relatively high content of particles with capsule particle morphology generally exhibit relatively high gloss. They may be blended with an anionically polymerized or free-radical-polymerized glass-clear or impact-modified polystyrene. To improve impact strength, they are preferably blended with anionically polymerized or free-radical-polymerized impact-modified polystyrene with cellular particle morphology.
Anionically polymerized, impact-modified polystyrene in which from 95 to 99 percent by volume of the disperse soft phase has capsule particle morphology and from 1 to 5 percent by volume has cellular particle morphology exhibits a property profile with a balance between gloss and impact strength.
These materials may be prepared directly by anionically polymerizing styrene in the presence of an anionic polymerization initiator and of a mixture of a styrene-butadiene two-block copolymer whose styrene block content is from 40 to 60% by weight, preferably from 45 to 55% by weight, and a styrene-butadiene-styrene three-block copolymer with a total styrene content of from 5 to 75% by weight, in particular from 25 to 50% by weight. It is also possible for the anionically polymerized, impact-modified polystyrene with cellular particle morphology to be mixed subsequently with the above-described styrene-butadiene-styrene three-block copolymer or with an impact-modified polystyrene with cellular particle morphology.
The styrene-butadiene block copolymers used have preferably been stopped with an alcohol or a phenol as chain terminator.
The residual butadiene content of the styrene-butadiene block copolymers used should be below 200 ppm, preferably below 50 ppm, in particular below 10 ppm.
The styrene-butadiene copolymer may be dissolved in styrene and, where appropriate, another solvent and used directly for the polymerization of styrene in the presence of the styrene-butadiene copolymer for preparing the impact-modified polystyrene.
The content of styrene-butadiene block copolymer, based on the impact-modified polystyrene, is advantageously from 5 to 25% by weight.
The conversion, based on styrene in the hard matrix, is generally above 90%, preferably above 99%. The process may in principle also be taken to complete conversion.
Instead of styrene, use may also be made of other vinylaromatic monomers for the polymerization of the hard matrix or of the styrene blocks in the block copolymers. Examples of others which are suitable are xcex1-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene, vinyltoluene, 1,2-diphenylethylene and 1,1-diphenylethylene, and mixtures. It is particularly preferable to use styrene.
Instead of butadiene, the rubbers may also contain other dienes, such as 1,3-pentadiene, 2,3-dimethylbutadiene, isoprene or mixtures of these.
The anionic polymerization initiators used are usually mono-, bi- or multifunctional alkyl, aryl or aralkyl compounds of alkali metals. It is advantageous to use organolithium compounds, such as ethyl-, propyl-, isopropyl-, n-butyl-, sec-butyl-, tert-butyl-, phenyl-, diphenylhexyl-, hexamethylenedi-, butadienyl-, isoprenyl-, or polystyryllithium, or the multifunctional compounds 1,4-dilithiobutane, 1,4-dilithio-2-butene or 1,4-dilithiobenzene. The amount needed of alkali metal organyl compound depends on the molecular weight desired, on the type and amount of the other metal organyl compounds used, and also on the polymerization temperature. It is generally from 0.002 to 5 mol percent, based on the total amount of monomer.
The polymerization may be carried out in the absence of or in the presence of a solvent. Solvents whose use is preferred are aromatic hydrocarbons or hydrocarbon mixtures, such as benzene, toluene, ethylbenzene, xylene or cumene. The use of toluene is particularly preferred.
The polymerization is preferably carried out at a solvent content below 40 percent by weight. The reaction rate here may be reduced by adding compounds which reduce the polymerization rate, known as retarders, as described in WO 98/07766. It is preferable for the retarder used to be magnesium organyl compounds, aluminum organyl compounds or zinc organyl compounds, alone or in mixtures.
Suitable magnesium organyl compounds are those of the formula R2Mg, where the radicals R, independently of one another, are hydrogen, halogen, C1-C20-alkyl or C6-C20-aryl. It is preferable to use dialkylmagnesium compounds, in particular the commercially available ethyl, propyl, butyl, hexyl or octyl compounds. It is particularly preferable to use (n-butyl)(sec-butyl)magnesium, which is soluble in hydrocarbons.
The aluminum organyl compounds used may be those of the formula R3Al, where the radicals R, independently of one another, are hydrogen, halogen, C1-C20-alkyl or C6-C20-aryl. Preferred aluminum organyl compounds are the trialkylaluminum compounds, such as triethylaluminum, triisobutylaluminum, tri-n-butylaluminum, triisopropylaluminum and tri-n-hexylaluminum. It is particularly preferable to use triisobutylaluminum. The aluminum organyl compounds used may also be those produced by partial or complete hydrolysis, alcoholysis, aminolysis or oxidation of alkyl- or arylaluminum compounds. Examples of these are diethylaluminum ethoxide, diisobutylaluminum ethoxide, diisobutyl(2,6-di-tert-butyl-4-methylphenoxy)aluminum (CAS No. 56252-56-3), methylaluminoxane, isobutylated methylaluminoxane, isobutylaluminoxane, tetraisobutyldialuminoxane and bis(diisobutyl)aluminum oxide.
The molar ratios of retarder to polymerization initiator may be varied within wide limits and depend mainly on the retarding effect desired, on the polymerization temperature, on the monomer composition and monomer concentration, and also on the molecular weight desired, and it is advantageous to select a molar ratio of retarder to polymerization initiator of from 0.2:1 to 10:1.
It is particularly preferable for the polymerization of the styrene to be carried out in the presence of a trialkylaluminum or dialkylmagnesium compound.
To increase the elongation at break, from 0.1 to 10% by weight, preferably from 0.5 to 5% by weight, of mineral oil, based on the impact-modified polystyrene, may be added in the process of the invention.
As described in WO 97/07766, the polymerization of the hard styrene matrix may be carried out batchwise or continuously, in stirred reactors, circulating reactors, tubular reactors, tower reactors or disc reactors. It is preferable for the polymerization to be carried out continuously in a reactor arrangement composed of at least one back-mixing reactor (e.g. stirred reactor) and of at least one non-back-mixing reactor (e.g. tower reactor).
After completion of the polymerization of the hard styrene matrix, it is preferable to carry out termination with a protic substance, for example alcohols, such as isopropanol, phenols, water or acids, such as aqueous carbon dioxide.
It can be advantageous to crosslink the rubber particles by controlling the temperature appropriately and/or by adding peroxides, in particular those with a high decomposition temperature, such as dicumyl peroxide. The peroxides are added here after completion of the polymerization and after any addition of a chain terminator, and prior to the devolatilization. However, it is preferable for the soft phase to be crosslinked thermally after the polymerization at from 200 to 300xc2x0 C.
Other conventional auxiliaries, such as stabilizers, lubricants, flame retardants, antistats, etc., may be added to the polymers of the invention.
The impact-modified polystyrene of the invention is suitable for producing fibers, films or moldings.